Hi all,
I’d like to share a side project: FXBox, which turns a Zynthian Kit V4 (Raspberry Pi 4, ZynScreen, ZynADAC) into a dedicated, standalone effects unit for external instruments. Plug in, switch on, play.
To be clear up front: it does not run the Zynthian software. It’s a separate, single-purpose setup on Raspberry Pi OS Lite 64-bit (Bookworm), for people who want to use the V4 hardware purely as a stompbox-style effects processor. It does build heavily on Zynthian’s work: the hardware configuration (MCP23017 mapping, ZynADAC setup, MIDI UART) comes straight from zynthian-sys, zyncoder and webconf, and the ZynScreen overlays are included unmodified. Many thanks to the Zynthian team for documenting all of that so well.
What it does
- Fixed chain of 8 effects, each on/off: Filter (5 types, LFO, envelope) → Saturation → Distortion (5 types, 2× oversampled) → Chorus → Phaser → Tape Echo → Reverb → Compressor/limiter
- Controlled entirely with the 4 encoders and S1–S4, no touchscreen needed
- Analogue-style VU needles plus dBFS peak/clip metering
- MIDI clock sync via DIN or USB, tap tempo, dotted/triplet divisions
- 44 factory presets, 32 user slots, last state restored at boot
- Click-free bypass, effect switching and preset changes, with tails/spill-over
How it’s built
C++ realtime engine on JACK with DSP from the Faust libraries (reverb is zita-rev1), and a Python + cairo UI drawing straight to the ZynScreen framebuffer. Three systemd services; the audio keeps running if the UI restarts.
Status
Early. It installs and runs on my V4: hardware detection, screen and controls work, and a first measurement showed 7 % DSP load and 0 xruns with four effects on (128 × 2 @ 48 kHz). Latency, boot time, full-chain load and DIN clock from real gear still need testing. The test report in the repo lists exactly what has and hasn’t been verified.
Repo: GitHub - Sreekmans/fxbox: Standalone multi-effect box for the Zynthian Kit V4: instrument input, effects (tape echo, reverb, chorus, phaser) out, MIDI clock sync and presets. · GitHub
Docs in English and Dutch. Note that the on-device UI is in Dutch for now.
Feedback, testers and ideas very welcome, especially from anyone with a V4 who’d like to try it, or with thoughts on effect choices.
Steven